Hypoxia protects against the neurotoxicity of kainic acid

Transcriptomic analysis of dorsal and ventral subiculum after induction of acute seizures by electric stimulation of the perforant pathway in rats

Preconditioning is a mechanism in which injuries induced by non-lethal hypoxia or seizures trigger cellular resistance to subsequent events. Norwood et al., in a 2010 study, showed that an 8-h-long period of electrical stimulation of the perforant pathway in rats is required for the induction of hippocampal sclerosis. However, in order to avoid generalized seizures, status epilepticus (SE), and death, a state of resistance to seizures must be induced in the hippocampus by a preconditioning paradigm consisting of two daily 30-min stimulation periods. Due to the importance of the subiculum in the hippocampal formation, this study aims to investigate differential gene expression patterns in the dorsal and ventral subiculum using RNA-sequencing, after induction of a preconditioning protocol by electrical stimulation of the perforant pathway. The dorsal (dSub) and ventral (vSub) subiculum regions were collected by laser-microdissection 24 h after preconditioning protocol induction in rats. RNA sequencing was performed in a Hiseq 4000 platform, reads were aligned using the STAR and DESEq2 statistics package was used to estimate gene expression. We identified 1176 differentially expressed genes comparing control to preconditioned subiculum regions, 204 genes were differentially expressed in dSub and 972 in vSub. The gene ontology enrichment analysis showed that the most significant common enrichment pathway considering up-regulated genes in dSub and vSub was steroid metabolism. In contrast, the most significant enrichment pathway considering down-regulated genes in vSub was axon guidance. Our results indicate that preconditioning induces changes in the expression of genes related to synaptic reorganization, increased cholesterol metabolism, and astrogliosis in both dSub and vSub. Both regions also presented a decrease in the expression of genes related to glutamatergic transmission and an increase in expression of genes related to complement system activation and GABAergic transmission. The down-regulation of proapoptotic and axon guidance genes in the ventral subiculum suggests that preconditioning may induce a neuroprotective environment in this region.

A Rationale for Hypoxic and Chemical Conditioning in Huntington's Disease

Neurodegenerative diseases are characterized by adverse cellular environments and pathological alterations causing neurodegeneration in distinct brain regions. This development is triggered or facilitated by conditions such as hypoxia, ischemia or inflammation and is associated with disruptions of fundamental cellular functions, including metabolic and ion homeostasis. Targeting intracellular downstream consequences to specifically reverse these pathological changes proved difficult to translate to clinical settings. Here, we discuss the potential of more holistic approaches with the purpose to re-establish a healthy cellular environment and to promote cellular resilience. We review the involvement of important molecular pathways (e.g., the sphingosine, δ-opioid receptor or N-Methyl-D-aspartate (NMDA) receptor pathways) in neuroprotective hypoxic conditioning effects and how these pathways can be targeted for chemical conditioning. Despite the present scarcity of knowledge on the efficacy of such approaches in neurodegeneration, the specific characteristics of Huntington’s disease may make it particularly amenable for such conditioning techniques. Not only do classical features of neurodegenerative diseases like mitochondrial dysfunction, oxidative stress and inflammation support this assumption, but also specific Huntington’s disease characteristics: a relatively young age of neurodegeneration, molecular overlap of related pathologies with hypoxic adaptations and sensitivity to brain hypoxia. The aim of this review is to discuss several molecular pathways in relation to hypoxic adaptations that have potential as drug targets in neurodegenerative diseases. We will extract the relevance for Huntington’s disease from this knowledge base.

Impaired hippocampal glucose metabolism during and after flurothyl-induced seizures in mice: Reduced phosphorylation coincides with reduced activity of pyruvate dehydrogenase

OBJECTIVE

To determine changes in glucose metabolism and the enzymes involved in the hippocampus ictally and postictally in the acute mouse flurothyl seizure model.

METHODS

[U-¹³ C]-Glucose was injected (i.p.) prior to, or following a 5 min flurothyl-induced seizure. Fifteen minutes later, mice were killed and the total metabolite levels and % ¹³ C enrichment were analyzed in the hippocampal formation using gas chromatography-mass spectrometry. Activities of key metabolic and antioxidant enzymes and the phosphorylation status of pyruvate dehydrogenase were measured, along with lipid peroxidation.

RESULTS

During seizures, total lactate levels increased 1.7-fold; however, [M + 3] enrichment of both lactate and alanine were reduced by 30% and 43%, respectively, along with a 28% decrease in phosphofructokinase activity. Postictally the % ¹³ C enrichments of all measured tricarboxylic acid (TCA) cycle intermediates and the amino acids were reduced by 46-93%. At this time, pyruvate dehydrogenase (PDH) activity was 56% of that measured in controls, and there was a 1.9-fold increase in the phosphorylation of PDH at ser232. Phosphorylation of PDH is known to decrease its activity.

SIGNIFICANCE

Here, we show that the increase of lactate levels during flurothyl seizures is from a source other than [U-¹³ C]-glucose, such as glycogen. Surprisingly, although we saw a reduction in phosphofructokinase activity during the seizure, metabolism of [U-¹³ C]-glucose into the TCA cycle seemed unaffected. Similar to our recent findings in the chronic phase of the pilocarpine model, postictally the metabolism of glucose by glycolysis and the TCA cycle was impaired along with reduced PDH activity. Although this decrease in activity may be a protective mechanism to reduce oxidative stress, which is observed in the flurothyl model, ATP is critical to the recovery of ion and neurotransmitter balance and return to normal brain function. Thus we identified promising novel strategies to enhance energy metabolism and recovery from seizures.

Effect of different mild hypoxia manipulations on kainic acid-induced seizures in the hippocampus of rats

The protective effect of the mild hypoxia to the epilepsy has been widely tested. Although it is found that the hypoxia protects the brain by up-regulation of hypoxia-inducible factor-1α, few focused on systematic comparisons between different mild hypoxia manipulations and their effects. The male Sprague-Dawley rats were observed following exposure to hypoxia before and after epilepsy for 3 days with 90 min per day. The effects of different mild hypoxia manipulations on kainic acid-induced epilepsy were compared from the perspective of morphology, molecular biology and behavioral test. Results showed that different mild hypoxia manipulations could inhibit the cell apoptosis of kainic acid-induced rat hippocampus and improve their physiological functions. The effect of preconditioning group was better than that of postconditioning group and that of preconditioning and postconditioning with mild hypoxia group was the best among all the groups. The result showed that the preconditioning and postconditioning of mild hypoxia was recommended pre- and post-epilepsy and exposure to mild hypoxia should be prolonged. These findings might provide new ideas and methods for the clinical treatment of epilepsy.

Pharmacological Preconditioning with GYKI 52466: A Prophylactic Approach to Neuroprotection

Some toxins and drugs can trigger lasting neuroprotective mechanisms that enable neurons to resist a subsequent severe insult. This “pharmacological preconditioning” has far-reaching implications for conditions in which blood flow to the brain is interrupted. We have previously shown that in vitro preconditioning with the AMPA receptor antagonist GYKI 52466 induces tolerance to kainic acid (KA) toxicity in hippocampus. This effect persists well after washout of the drug and may be mediated via inverse agonism of G-protein coupled receptors (GPCRs). Given the amplifying nature of metabotropic modulation, we hypothesized that GYKI 52466 may be effective in reducing seizure severity at doses well below those normally associated with adverse side effects. Here we report that pharmacological preconditioning with low-dose GYKI imparts a significant protection against KA-induced seizures in vivo. GYKI (3 mg/kg, s.c.), 90–180 min prior to high-dose KA, markedly reduced seizure scores, virtually abolished all level 3 and level 4 seizures, and completely suppressed KA-induced hippocampal c-FOS expression. In addition, preconditioned animals exhibited significant reductions in high frequency/high amplitude spiking and ECoG power in the delta, theta, alpha, and beta bands during KA. Adverse behaviors often associated with higher doses of GYKI were not evident during preconditioning. The fact that GYKI is effective at doses well-below, and at pre-administration intervals well-beyond previous studies, suggests that a classical blockade of ionotropic AMPA receptors does not underlie anticonvulsant effects. Low-dose GYKI preconditioning may represent a novel, prophylactic strategy for neuroprotection in a field almost completely devoid of effective pharmaceuticals.

Animal model of the short-term cardiorespiratory effects of intermittent vagus nerve stimulation

PURPOSE

To develop an animal model of the effects of vagus nerve stimulation (VNS) on heart rate and respiration in studies of seizure treatment.

METHODS

Nine rats implanted with ECG, EMG, and VNS electrodes and pulse generator were stimulated with 81 different sets of parameters while they slept in a plethysmographic box.

RESULT

From cardiorespiratory effects of VNS, an index (alpha) was found to distinguish between weak and strong VNS doses. Weak VNS dose induced an increase in respiratory frequency and no significant change in heart rate. The effect of VNS on respiration, similar to that observed in children, can be divided into 3 phases. Strong VNS dose induced a decrease in respiratory frequency concomitant with a decrease in heart rate. Increasing the intensity of the VNS induced a proportional increase in the maximal inspiratory strength.

CONCLUSION

Various VNS parameter settings induce different and concomitant cardiorespiratory variations in conscious sleeping rats. These effects correlate with the intensity of the VNS parameters. Understanding the effects of the intensity of VNS parameters may allow for further optimization of VNS parameters in patients receiving VNS.

Inhibitory effect of vasopressin receptor antagonist OPC-31260 on experimental brain oedema induced by global cerebral ischaemia

The effects of the non-peptide vasopressin V(2) receptor antagonist 5-dimethylamino-1-[4-(2-methylbenzoylamino)benzoyl]-2,3,4,5-tetrahydro-1H-benzazepine hydrochloride (OPC-31260) on the cerebral oedema induced by general cerebral hypoxia were studied in rats. The general cerebral hypoxia was produced by bilateral common carotid ligation in Sprague-Dawley rats of the CFY strain. By 6 h after the ligation, half of the rats had died, but the survival rate was significantly higher following OPC-31260 administration. Electron microscopic examinations revealed typical ischaemic changes after the carotid ligation. The carotid ligation increased the brain contents of water and Na(+) and enhanced the plasma vasopressin level. The increased brain water and Na(+) accumulation was prevented by OPC-31260 administration, but the plasma vasopressin level was further enhanced by OPC-31260. These results demonstrate the important role of vasopressin in the development of the disturbances in brain water and electrolyte balance in response to general cerebral hypoxia. The carotid ligation-induced cerebral oedema was significantly reduced following oral OPC-31260 administration. The protective mechanism exerted by OPC-31260 stems from its influence on the renal vasopressin V(2) receptors. These observations might suggest an effective approach to the treatment of global hypoxia-induced cerebral oedema in humans.

Molecular physiology of preconditioning-induced brain tolerance to ischemia

Ischemic tolerance describes the adaptive biological response of cells and organs that is initiated by preconditioning (i.e., exposure to stressor of mild severity) and the associated period during which their resistance to ischemia is markedly increased. This topic is attracting much attention because preconditioning-induced ischemic tolerance is an effective experimental probe to understand how the brain protects itself. This review is focused on the molecular and related functional changes that are associated with, and may contribute to, brain ischemic tolerance. When the tolerant brain is subjected to ischemia, the resulting insult severity (i.e., residual blood flow, disruption of cellular transmembrane gradients) appears to be the same as in the naive brain, but the ensuing lesion is substantially reduced. This suggests that the adaptive changes in the tolerant brain may be primarily directed against postischemic and delayed processes that contribute to ischemic damage, but adaptive changes that are beneficial during the subsequent test insult cannot be ruled out. It has become clear that multiple effectors contribute to ischemic tolerance, including: 1) activation of fundamental cellular defense mechanisms such as antioxidant systems, heat shock proteins, and cell death/survival determinants; 2) responses at tissue level, especially reduced inflammatory responsiveness; and 3) a shift of the neuronal excitatory/inhibitory balance toward inhibition. Accordingly, an improved knowledge of preconditioning/ischemic tolerance should help us to identify neuroprotective strategies that are similar in nature to combination therapy, hence potentially capable of suppressing the multiple, parallel pathophysiological events that cause ischemic brain damage.

Domoic acid preconditioning and seizure induction in young and aged rats

Clinical reports suggest that the elderly are hypersensitive to the neurological effects of domoic acid (DOM). In the present study we assessed DOM-induced seizures in young and aged rats, and seizure attenuation following low-dose DOM pretreatment (i.e. preconditioning). Seizure behaviours following saline or DOM administration (0.5-2mg/kg i.p.) were continuously monitored for 2.5h in naïve and DOM preconditioned rats. Competitive ELISA was used to determine serum and brain DOM concentrations. Dose- and age-dependent increases in seizure activity were evident in response to DOM. Lower doses of DOM in young and aged rats promoted low level seizure behaviours. Animals administered high doses (2mg/kg in young; 1mg/kg in aged) progressed through various stages of stereotypical behaviour (e.g., head tics, scratching, wet dog shakes) before ultimately exhibiting tonic-clonic convulsions. Serum and brain DOM analysis indicated impaired renal clearance as contributory to increased DOM sensitivity in aged animals, and this was supported by seizure analysis following direct intrahippocampal administration of DOM. Preconditioning young and aged animals with low-dose DOM 45-90 min before high-dose DOM significantly reduced seizure intensity. We conclude that age-related supersensitivity to DOM is related to reduced clearance rather than increased neuronal sensitivity, and that preconditioning mechanisms underlying an inducible tolerance to excitotoxins are robustly expressed in both young and aged CNS.

Gene expression changes after seizure preconditioning in the three major hippocampal cell layers

Rodents experience hippocampal damage after status epilepticus (SE) mainly in pyramidal cells while sparing the dentate granule cell layer (DGCL). Hippocampal damage was prevented in rats that had been preconditioned by brief seizures on 2 consecutive days before SE. To identify neuroprotective genes and biochemical pathways changed after preconditioning we compared the effect of preconditioning on gene expression in the CA1 and CA3 pyramidal and DGCLs, harvested by laser capture microscopy. In the DGCL the expression of 632 genes was altered, compared to only 151 and 58 genes in CA1 and CA3 pyramidal cell layers. Most of the differentially expressed genes regulate tissue structure and intra- and extracellular signaling, including neurotransmission. A selective upregulation of energy metabolism transcripts occurred in CA1 pyramidal cells relative to the DGCL. These results reveal a broad transcriptional response of the DGCL to preconditioning, and suggest several mechanisms underlying the neuroprotective effect of preconditioning seizures.

Kainic acid-induced oxidative injury is attenuated by hypoxic preconditioning

Female Wistar rats were subjected to 380 mmHg in an altitude chamber for 15 h/day for 28 days. Hypoxic preconditioning attenuated kainic acid (KA)-induced oxidative injury, including KA-elevated lipid peroxidation and neuronal loss in rat hippocampus. Furthermore, KA-induced translocation of cytochrome c and apoptosis-inducing factor from mitochondria to cytosol was attenuated in the hypoxic rats. In addition, hypoxic preconditioning attenuated the KA-induced reduction in glutathione content and superoxide dismutase as well as KA-induced increase in glutathione peroxidase. Although local infusion of KA increased hippocampal NF-kappaB binding activity in the normoxic rat, hypoxia further enhanced KA-elevated NF-kappaB binding activity. Moreover, hypoxic preconditioning potentiated the KA-induced increase in Bcl-2 level in the lesioned hippocampus. Our data suggest that hypoxic preconditioning exerts its neuroprotection of KA-induced oxidative injury via enhancing NF-kappaB activation, upregulating the antioxidative defense system, and attenuating the apoptotic process.

Chronic hypobaric hypoxia induces tolerance to acute hypoxia and up-regulation in alpha-2 adrenoceptor in rat locus coeruleus

Hypoxia preconditioning has been shown to produce tolerance against brain injuries. The hypothesis of this study is that chronic hypobaric hypoxia may also induce acute hypoxia tolerance. We used intracellular recording in slices from rats exposed to chronic hypobaric hypoxia (exposed) and control to investigate the effects of chronic hypobaric hypoxia on the physiology of locus coeruleus (LC) including neuronal excitability. The results showed 35.7% reduced spontaneous firing rate and no change for membrane potential and input resistance in exposed neurons. In response to the alpha-2 adrenoceptor (A2R) agonist clonidine, both the hyperpolarizing potency and efficacy were increased indicated by a decreased EC(50) (control: 30.9 nM and exposed: 19.7 nM) and a 50.5% increase in maximum hyperpolarized potential, respectively. A2R binding sites were also increased 21% in exposed neurons measured by radioligand [(3)H]rauwolscine binding assay. When treated with acute N(2)-hypoxia, the cell survival time (ST) was longer in exposed neurons, suggesting that a tolerance was induced. In addition, the ST for both groups of LC neurons was decreased by the A2R antagonist yohimbine and increased by the glutamate receptor antagonist kynurenic acid but not by MK-801; the decreased percentage of ST by yohimbine was larger and the increased percentage by kynurenic acid was smaller in exposed neurons. The results suggested that up-regulation of A2R and altered non-NMDA glutamate receptor function induced by chronic hypobaric hypoxia may underlie, in part, the decreased LC neuronal excitability and acute hypoxia tolerance.

Vagus Nerve Stimulation Induces Concomitant Respiratory Alterations and a Decrease in SaO2 in Children

PURPOSE

To analyze respiratory alterations and effects on SaO(2) caused by vagus nerve stimulation (VNS) in children with epilepsy.

METHODS

Polysomnographic recordings, including electroencephalography, thoracoabdominal distention, nasal airflow, SaO(2), and VNS artifact were evaluated in 10 children with pharmacoresistant epilepsy treated with VNS.

RESULTS

Each VNS caused a significant increase in respiratory frequency (p < 0.05) throughout the stimulation period and a decrease in thoracoabdominal-distention amplitude (p < 0.05), especially at the beginning of the stimulation. These respiratory alterations induced a decrease in SaO(2) from 1 to 5%. The effects of VNS on respiration differed significantly between rapid-eye-movement (REM) and non-REM (NREM) sleep states.

CONCLUSIONS

VNS caused a pronounced change in respiration in children with epilepsy, and this induced a decrease in SaO(2). It is possible that VNS has a neuroprotective effect, and this possibility calls for further investigation.

Protecting Neurons

Brain injury evolves over time, often taking days or even weeks to fully develop. It is a dynamic process that involves immediate oxidative stress and excitotoxicity followed by inflammation and preprogrammed cell death. This article presents a brief overview of mechanisms of neuroprotection in the developing brain. Although the focus is on ischemic injury, the conclusions drawn apply to any type of brain insult-epileptic seizures, trauma, or ischemia. Strategies of neuroprotection include salvaging neurons through the use of targeted pharmacotherapies, protecting neurons through preconditioning, and repairing neurons by enhancing neurogenesis. Drug therapies that dampen the impact of immediate and downstream postinjury events are only modestly effective in protecting the brain from ischemic injury. In experimental models, complete or true protection can be achieved only through preconditioning, a process during which an animal develops tolerance to an otherwise lethal stressor. Although of no clinical use, preconditioning models have provided valuable insight into how repair systems work in the brain. Cumulative evidence indicates that the same genes that are upregulated during preconditioning, those mediating true protection, are also upregulated during injury and repair. Specifically, hypoxic preconditioning and hypoxic-ischemic insult have been shown to induce hypoxia inducible factor-1 (HIF-1) and its target survival genes, vascular endothelial growth factor (VEGF), and erythropoietin (Epo) in rodents. Of particular interest is the upregulation of Epo, a growth factor that may have therapeutic potential in the treatment of ischemic stroke. At this time, however, the postinjury enhancement of neurogenesis appears to offer the best hope for long-lasting functional recovery following brain injury.

Hypoxic preconditioning attenuated in kainic acid-induced neurotoxicity in rat hippocampus

The neuroprotective effect of hypoxic preconditioning on kainate (KA)-induced neurotoxicity, including apoptosis and necrosis, was investigated in rat hippocampus. Female Wistar-Kyoto rats were subjected to 380 mm Hg in an altitude chamber for 15 h/day for 28 days. Intrahippocampal infusion of KA was performed in chloral hydrate anesthetized rats, which acutely elevated 2,3-dihydroxybenzoic acid levels in normoxic rats. Seven days after the infusion, KA increased lipid peroxidation in the infused hippocampus and resulted in hippocampal CA3 neuronal loss. A 4-week hypoxic preconditioning attenuated KA-induced elevation in hydroxyl radical formation and lipid peroxidation as well as KA-induced neuronal loss. The effects of hypoxic preconditioning on KA-induced apoptosis and necrosis were investigated further. Two hours after KA infusion, cytosolic cytochrome c content was increased in the infused hippocampus. Twenty-four hours after KA infusion, pyknotic nuclei, cellular shrinkage, and cytoplasmic disintegration, but not TUNEL-positive staining, were observed in the CA3 region of hippocampus. Forty-eight hours after KA infusion, both DNA smear and DNA fragmentation were demonstrated in the infused hippocampus. Furthermore, TUNEL-positive cells, indicative of apoptosis, in the infused hippocampus were detected 72 h after KA infusion. Hypoxic pretreatment significantly reduced necrotic-like events in the KA-infused hippocampus. Moreover, hypoxic preconditioning attenuated apoptosis induced by KA infusion, including elevation in cytosolic cytochrome c content, TUNEL-positive cells, and DNA fragmentation. Our data suggest that hypoxic preconditioning may exert its neuroprotection of KA-induced oxidative injuries via attenuating both apoptosis and necrosis in rat hippocampus.

Transient hyperthermia protects against subsequent seizures and epilepsy-induced cell damage in the rat

Many mild preconditioning stress conditions, including physical and metabolic injuries, increase the resistance of neurons to subsequent more severe stresses of the same or different type. This "tolerance phenomenon" lasts one to several weeks, providing a unique opportunity to investigate endogenous neuroprotective mechanisms. The aim of this study was to find a physiological and easily applicable preconditioning stimulus able to confer protection against convulsant-induced neuronal damage and seizures. We found that moderate transient hyperthermic preconditioning markedly reduced kainic-acid-induced neuronal cell loss and attenuated susceptibility to bicuculline-induced seizures. Prevention of cell damage (approximately 50%) was efficient both in vitro in organotypic hippocampal slice cultures and in vivo in adult rats. This protection lasted about 1 week and peaked 3 to 5 days after pretreatment. Unraveling the mechanisms of heat shock preconditioning-induced protection against epilepsy should lead to the development of new therapeutic strategies.

La tolérance cérébrale : un choix prometteur vers de nouvelles thérapies contre les maladies neurologiques

Ischemia and seizures are common diseases that result in neuronal death. To-date, there are no available treatments to block or reverse neuronal death pathways in patients who suffer from these diseases. All drugs that have been shown to be neuroprotective in animal models have failed in human trials. Therefore, the potential of preventative strategies for therapy is increasingly explored. Experimental studies have demonstrated that a brief cerebral ischemic insult, that is not harmful by itself, results in a temporary protective adaptation in the brain against a subsequent ischemic episode that would otherwise be lethal. This process, termed ischemic preconditioning, has been confirmed in different models of cerebral ischemia. A similar phenomenon observed after a mild epileptic insult conferred a transitory tolerance to a subsequent epileptic episode. This process is termed epileptic tolerance. Other stresses, like hyperthermia or spreading depression, also enhanced brain resistance to detrimental effects of ischemic or epileptic injury. Recently, a cross tolerance between ischemia and epilepsy has been reported. Also, some retrospective studies in humans suggest that endogenous ischemic preconditioning exists in the brain. Altogether these insights of brain tolerance point to the future discovery of potentially useful targets for acute neuroprotection as well as preventive therapy.

Transforming growth factor-beta 1-mediated neuroprotection against excitotoxic injury in vivo

Ischemic preconditioning is a phenomenon that describes how a sublethal ischemic insult can induce tolerance to subsequent ischemia. This phenomenon has been observed after focal or global ischemia in different animal models. However, the hypothesis that bacterial infection might lead to neuronal tolerance to injury has not been investigated. To mimic cerebral bacterial infection, we injected bacterial lipopolysaccharide (LPS) in the right dorsal hippocampus, followed 24 hours later by an excitotoxic lesion using kainic acid in the mouse model. Quantification of lesion size after cresyl violet counterstaining revealed that LPS pretreatment afforded neuroprotection to CA3 neurons against KA challenge. To investigate the events underlying this protection, we studied the cytokine profile induced after LPS injection. Interleukin (IL)-1 beta and transforming growth factor beta 1 (TGF-beta 1) were the main cytokines expressed at 24 hours after LPS injection. Because IL-1 beta has been described as deleterious in acute injury, we decided to investigate the function of TGF-beta 1. An adenovirus expressing a constitutively active form of TGF-beta 1 was injected intracerebrally 1 week before the induction of excitotoxic lesion, and neuronal protection was observed. To confirm the neuroprotective role of TGF-beta 1, the TGF-beta 1 adenovirus was replaced by recombinant human TGF-beta 1 protein and total neuroprotection was observed. Furthermore, the antibody-mediated blocking of TGF-beta 1 action prevented the protective effect of pretreatment with LPS. We have demonstrated in vivo that the cerebral tolerance phenomenon induced by LPS pretreatment is mediated by TGF-beta 1 cytokine.

Ischemic tolerance

A brief period of cerebral ischemia confers transient tolerance to a subsequent ischemic challenge in the brain. This phenomenon of ischemic tolerance has been confirmed in various animal models of forebrain ischemia and focal cerebral ischemia. Since the ischemic tolerance afforded by preceding ischemia can bring about robust protection of the brain, the mechanism of tolerance induction has been extensively studied. It has been elucidated that ischemic tolerance protects neurons, and at the same time, it preserves brain function. Further experiments have shown that metabolic and physical stresses can also induce cross-tolerance to cerebral ischemia, but the protection by cross-tolerance is relatively modest. The underlying mechanism of ischemic tolerance still is not fully understood. Potential mechanisms may be divided into two categories: (1) A cellular defense function against ischemia may be enhanced by the mechanisms inherent to neurons. They may arise by posttranslational modification of proteins or by expression of new proteins via a signal transduction system to the nucleus. These cascades of events may strengthen the influence of survival factors or may inhibit apoptosis. (2) A cellular stress response and synthesis of stress proteins may lead to an increased capacity for health maintenance inside the cell. These proteins work as cellular "chaperones" by unfolding misfolded cellular proteins and helping the cell to dispose of unneeded denatured proteins. Recent experimental data have demonstrated the importance of the processing of unfolded proteins for cell survival and cell death. The brain may be protected from ischemia by using multiple mechanisms that are available for cellular survival. If tolerance induction can be manipulated and accelerated by a drug treatment that is safe and effective enough, it could greatly improve the treatment of stroke.

Normobaric hypoxia induces tolerance to focal permanent cerebral ischemia in association with an increased expression of hypoxia-inducible factor-1 and its target genes, erythropoietin and VEGF, in the adult mouse brain

Tolerance to cerebral ischemia is achieved by preconditioning sublethal stresses, such as ischemia or hypoxia, paradigms in which the decrease of O2 availability may constitute an early signal inducing tolerance. In accordance with this concept, this study shows that hypoxia induces tolerance against focal permanent ischemia in adult mice. Normobaric hypoxia (8% O2 of 1-hour, 3-hour, or 6-hour duration), performed 24 hours before ischemia, reduces infarct volume by approximately 30% when compared with controls. To elucidate the mechanisms underlying this neuroprotection, the authors investigated the effects of preconditioning on cerebral expression of hypoxia-inducible factor-1alpha (HIF-1alpha) and its target genes, erythropoietin and vascular endothelial growth factor (VEGF). Hypoxia, whatever its duration (1 hour, 3 hours, 6 hours), rapidly increases the nuclear content of HIF-1alpha as well as the mRNA levels of erythropoietin and VEGF. Furthermore, erythropoietin and VEGF are upregulated at the protein level 24 hours after 6 hours of hypoxia. The authors' findings show that (1) hypoxia elicits a delayed, short-lasting (<72 hours) tolerance to focal permanent ischemia in the adult mouse brain; (2) HIF-1 target genes could contribute to the establishment of tolerance; and (3) this model might be a useful paradigm to further study the mechanisms of ischemic tolerance, to identify new therapeutic targets for stroke.

Stress response to hypoxia in gerbil brain: HO-1 and Mn SOD expression and glial activation

Hypoxic preconditioning has been shown to induce neuroprotection against a subsequent damaging insult. In order to study the underlying molecular and cellular mechanisms of hypoxic preconditioning, we investigated, in gerbil hippocampus, the effects in vivo of transient exposure to hypoxia (4% O(2) for 6 min followed by either 48 h or 7 days of reoxygenation) (i) on the induction of 72 kDa heat shock protein (HSP72), heme oxygenase-1 (HO-1) and manganese superoxide dismutase (Mn SOD) as assessed by Western immunoblotting and (ii) on the astroglial and microglial activation as detected by both immunohistochemistry and Western immunoblotting for GFAP, and histochemistry for isolectin B4, respectively. Our data show that, although hypoxia and subsequent reoxygenation led to neither neuronal damage nor HSP72 induction in gerbil hippocampus, it induced a progressive and sustained expression of HO-1 and Mn SOD. As expected from the absence of neuronal death, hypoxia was not associated with microglial activation but led to a significant astrocytic activation. These findings demonstrate that transient hypoxia enhances the antioxidative enzymatic defenses of the brain, which are susceptible to increased tolerance against a subsequent damaging insult.

Does fucose or piracetam modify the effect of hypoxia preconditioning against pentylenetetrazol-induced seizures?

To clarify the question whether the duration of hypoxia exposure has an influence on the point in time or the strength of hypoxic preconditioning, hypoxia exposure of rats lasting 1 and 8 h was tested regarding the modification of susceptibility to acute pentylenetetrazol-induced seizures. Following the short-lasting (1 h) hypoxia, the maximum level of preconditioning action was observed 7 days after hypoxia, whereas the longer-lasting hypoxia (8 h) produced the maximum level of protection 14 days after hypoxia. We investigated the influence of fucose and piracetam on the effect of hypoxia preconditioning by the application of the substances 20 min before the beginning of hypoxia exposure. Fucose did not modify the result of hypoxia preconditioning. But after the treatment with piracetam, the preconditioning effect was prevented following hypoxia lasting 1 and 8 h. We suggest that the radical scavenger properties of piracetam are responsible for the absence of protection against pentylenetetrazol-evoked seizures.

The role of opioid receptors in hypoxic preconditioning against seizures in brain

Preconditioning is defined as an adaptive mechanism produced by short periods of hypoxia/ischemia, resulting in protection against subsequent ischemic insult, and development of seizures. Results of the present study demonstrate that an episode of normobar hypoxia reduces the susceptibility to convulsions induced by pentylenetetrazol (PTZ) 30 min, 24 h, as well as 4 and 7 days later. Administration of morphine showed similar effects after 24 h. Naloxone, given before ischemic preconditioning, as well as morphine, blocked the development of the protection. Administration of D-Ala-Met-enkephalin-Gly-ol (DAMGO - a selective mu-opioid receptor agonist), as well as trans-3, 4-dichloro-N-methyl-N-[7-(1-pyrrolidinyl) cycloexilbenzeneacetamide ethane sulfonate] (U-69,593 - a selective kappa-opioid receptor agonist), mimicked the effects of hypoxic preconditioning (HPC). (-)-N-(Cyclopropylmethyl)-4,14-dimethoxymorphinan-6-one (cyprodime - a selective mu-opioid receptor antagonist, as well as nor-binaltorphimine dihydrochloride (nor-BNI - selective kappa-opioid receptors antagonist), given before HPC as well as before respective opioid receptor agonists, blocked the development of the protection. This study provides evidence that mu- and kappa-opioid receptors are involved in HPC against seizures in the brain.

The importance of free hydroxyl radicals to hypoxia preconditioning

Hypoxia preconditioning states that a sublethal hypoxia period will afford neuroprotection against a second harmful event. In our experiments, we carried out a procedure for the development of hypoxia preconditioning in adult male Wistar rats using hypoxic exposure (9% O(2); 91% N(2)) for 1 h. The protection against pentylenetetrazol (PTZ)-induced seizures was studied. For this, rats were tested by a single injection of PTZ (55 mg/kg i.p.) on days 1-21 after hypoxia exposure. The hypoxia exposure significantly prevented the development of acute PTZ convulsion at different times after hypoxia. The present study was designed to determine the effect of N-t-butyl-alpha-phenylnitrone (PBN), an electron-trapping agent and free radical scavenger, on hypoxia preconditioning against PTZ seizures 7 days after hypoxia exposure. PBN abolished the protective action of hypoxia exposure. The generation of free hydroxyl radicals in the brains of animals exposed to hypoxia was determined in a second experiment. For this purpose, the rats were i. p. pretreated with 30 mg/kg PBN and NaCl, respectively, 20 min before the start of hypoxia exposure. Forty-five minutes later the rats were i.p. injected with 300 mg/kg sodium salicylate and once again exposed to hypoxia for 15 min. Immediately after that the animals were decapitated and the free hydroxyl radicals and the salicylate content were estimated in the whole brain without cerebellum. Hypoxia preconditioned animals pretreated with NaCl showed a significantly higher extent of free hydroxyl radicals in the brain compared with PBN-injected preconditioned animals and with naive and sham exposed controls. The results pointed out that the generation of free reactive oxygen species under hypoxic conditions in the brain is involved in the development of the hypoxic preconditioning phenomenon.

A global hypoxia preconditioning model: neuroprotection against seizure-induced specific gravity changes (edema) and brain damage in rats

Hypoxia preconditioning states that a sublethal hypoxia episode will afford neuroprotection against a second challenge in the near future. We describe and discuss a procedure for the development of global hypoxia preconditioning in adult male Wistar rats, using a mildly hypoxic (9% O(2), 91% N(2)) atmospheric exposure of 8 h. The persistence of neuroprotection was analyzed using a kainic acid (KA) model of brain injury. Rats were challenged with KA (14 mg/kg, i.p.) on 1-14 days post-hypoxia. The effects of hypoxia preconditioning on seizure score, weight loss, brain edema and histopathology were assessed. Brain edema, predominantly of vasogenic origin, was measured 24 h after KA administration using a reproducible and quantitative method based on the specific gravities of tissue samples. A density gradient column (1.0250-1.0650 g/cm(3)) comprised of kerosene and bromobenzene was used to assess the presence of edema in regions involved in seizure initiation and propagation that are normally extensively damaged (i.e., piriform cortex and hippocampus). Specific gravities of tissues were calculated through extrapolation with known NaCl standards. We found that hypoxia preconditioning prevented the formation of edema in these brain regions when KA challenge was given 1, 3, and 7, but not 14 days post-hypoxia exposure. Furthermore, neuroprotection was observed in animals that had robust seizures. The described procedure may be used to examine the neuroprotective mechanisms induced by global hypoxia preconditioning against many subsequent challenges reflecting a variety of experimental models of brain injury, and will provide a better understanding of the brain response to hypoxia and stress.

Hypoxia preconditioning attenuates brain edema associated with kainic acid-induced status epilepticus in rats

Kainic acid (KA)-induced seizures elicit edema associated with necrosis in susceptible brain regions (e.g., piriform cortex and hippocampal CA1 and CA3 regions). To test the hypothesis that hypoxia preconditioning protects against KA-induced edema formation, adult male rats were exposed to a 9% O2, 91% N2 atmosphere for 8 h. KA (14 mg/kg, i.p.) was administered 1, 3, 7, or 14 days later. Regional analysis of edema indicated that hypoxia exposure attenuated edema formation in piriform and frontal cortices and hippocampus when KA was given 1, 3, or 7 days later but not 14 days after hypoxia. Cycloheximide (2 mg/kg s.c.) given 1 h prior to hypoxia prevented the protective effect of hypoxia on KA-induced edema attenuation in the piriform cortex and hippocampus. Thus, hypoxic challenge induces a general adaptive response that protects against the seizure-associated pathophysiology, with no direct relationship to seizure intensity. This response may involve stress-related transcription factors and effector proteins.

Anoxia during kainate status epilepticus shortens behavioral convulsions but generates hippocampal neuron loss and supragranular mossy fiber sprouting

In rats, this study determined the impact of systemic hypoxia during late kainate-induced status epilepticus on hippocampal neuron loss and mossy fiber sprouting. Non-fasted Sprague Dawley rats were prepared as follows: Naive controls (n=5); rats placed 2 min in a hypoxia chamber (hypoxia only; n=6); rats that seized for more than 6 h from kainic acid (KA-status; 12 mg/kg; i.p.; n=7); and another KA-status group placed into the hypoxia chamber 75 min after the convulsions started (KA-status/hypoxia; n=16). All rats, except for half of the KA-status/hypoxia animals, were perfused 2 weeks later (short-term). The other 8 KA-status/hypoxia rats were perfused after 2 months (long-term). Hippocampal sections were studied for neuron densities and aberrant mossy fiber sprouting at three ventral to dorsal levels. Fascia dentata (FD) mossy fiber sprouting was quantified as an increase in the inner minus outer molecular layer (IML-OML) gray value (GV) difference. Behaviorally, KA-status/hypoxia rats had a shorter duration of convulsive status epilepticus than KA-status animals without anoxia. Hippocampal sections showed that compared to controls: (1) hypoxia-only rats showed no differences in ventral neuron densities and neo-Timm's stained IML-OML GVs; (2) KA-status rats had decreased CA3 densities and a non-significant increase in ventral IML-OML GV differences; and (3) KA-status/hypoxia short-term animals showed decreased hilar, CA3 and CA1 densities and increased ventral IML-OML GV differences. Compared to KA-status/hypoxia short-term rats, long-term animals showed no differences in ventral hippocampal neuron densities, but middle and dorsal sections demonstrated increased IML-OML GV differences and animals were observed to have spontaneous limbic epilepsy. These results indicate that rats exposed to kainate-induced status epilepticus for over 1 h and then a hypoxic insult had a shorter duration of convulsive status, decreased hippocampal neuron densities and greater FD mossy fiber sprouting than controls and the amount of neuronal damage and sprouting was slightly more than animals subjected to 6 h of kainate-induced status. This supports the hypothesis that a physiologic insult during status can shorten the convulsive episode, but still produce hippocampal pathology with a number of clinical and pathologic similarities to human mesial temporal lobe epilepsy (MTLE).

Piracetam prevents pentylenetetrazol kindling-induced neuronal loss and learning deficits

The effect of the nootropic drug piracetam (100 mg/kg) on kindled seizures, kindling-induced learning deficits, and histological alterations due to changes in central excitability was investigated in Wistar rats. The animals were kindled by repeated i.p. injections of an initially subconvulsive dose of pentylenetetrazol (PTZ). As a control, piracetam or physiological saline was given 60 minutes before PTZ. Twenty-four hours after completion of kindling the rats were tested in a shuttle-box paradigm. Seven days after the final kindling injection, the animals received a challenge dose of PTZ. Finally, the brains of the rats were processed for histological investigation. Pentylenetetrazol-kindled animals showed increasing seizure scores, and a learning deficit in the shuttle-box. Piracetam had no effect either on kindling development or on the reaction to a challenge dose of PTZ, but it protected the animals against the kindling-induced reduction of learning performance. The substance had no effect on learning performance in control animals. In distinct hippocampal structures, a neuronal cell loss was found in kindled rats. Interestingly, piracetam counteracted this damage efficaciously. The effects of piracetam are discussed in terms of its cytoprotective action. It is suggested that a coadministration of piracetam with clinically used antiepileptic drugs might be useful in antiepileptic therapy.

Induction of tolerance and mossy fibre neuropeptide-Y expression in the contralateral hippocampus following a unilateral intrahippocampal kainic acid injection in the rat

We have previously reported an ectopic expression of neuropeptide-Y (NPY) immunoreactivity in mossy fibres (MFs) in the contralateral hippocampus following a unilateral intrahippocampal (IH) injection of kainic acid (KA). In the present study we report that, in addition to MF NPY expression, unilateral IH KA injections also induce tolerance towards a subsequent intracerebroventricular (ICV) contralateral KA injection, resulting in a reduction in the number of overt seizures and degree of cell loss.

Gradation of kainic acid-induced rat limbic seizures and expression of hippocampal heat shock protein-70

Systemic injection of kainic acid (KA) induces limbic seizures in rats, which resemble human temporal lobe epilepsy, the most common form of adult human epilepsy. In this study, we have investigated KA-elicited limbic seizures in the rats by correlating the severity of the seizure attacks with the expression of hippocampal heat shock protein-70 (HSP70) which has been suggested to be a marker for neuronal injury/death in this model of seizures. After a systemic injection of KA, six stages of limbic seizures have been classified, namely, staring (stage 1), wet dog shake (stage 2), hyperactivity (stage 3), rearing (stage 4), rearing and falling (stage 5), and jumping (stage 6). Stages 4, 5 and 6 were further divided into mild and severe sub-stages. HSP70 expression was not detected in animals with stages 1 and 2 seizures. At stage 3 a small amount of HSP70 immunoreactive neurons was detected in the CA3 field and the dentate hilus. From stage 4 to stage 5 the degree of HSP70 immunoreactivity increased in the CA1 field from a few positive cells in stage 4 mild to large numbers of immunoreactive neurons in stage 5 severe. HSP70 became detectable in pyramidal cells in the CA2 field from stage 5 severe and higher. In animals with stage 6 seizures, the majority of HSP70 expression became located in glial cells throughout the whole hippocampus. We concluded that HSP70 expression in the hippocampus positively correlates with the severity of KA-elicited limbic seizures.

The effects of perinatal hypoxia on the behavioral, neurochemical, and neurohistological toxicity of the metabolic inhibitor 3-nitropropionic acid

3-nitropropionic acid (3-NPA) neurotoxicity and long-term effects of perinatal hypoxia were evaluated in 18 adult rats. Hypoxia-insulted (I) and noninsulted (NI) rats were delivered by cesarean section. Hypoxic insult was effected by submerging dissected uterine horns in warmed saline for 15 min. NI rats were delivered from the adjacent nonsubmerged horns. At postnatal day 90, I and NI rats were trained to perform tasks thought to measure behaviors dependent upon aspects of time estimation (TE), motivation, and learning. At 12 months of age, rats were injected i.p. with escalating doses of 3-NPA (5 mg/kg/day to a maximum of 30 mg/kg/day) immediately after each test session and sacrificed at the end of treatment. Additional male rats were used as untreated controls. Although 3-NPA produced a dose-dependent impairment of performance in each task, the effects were qualitatively similar for each group. A significant difference between I and NI rats was, however, observed in the TE task where NI rats completed less of the task at high doses of 3-NPA compared to I rats. Compared to untreated controls, dopamine concentrations were decreased in caudate nucleus of both I and NI rats after 3-NPA. Specific areas most frequently damaged included cerebral cortex, hippocampal subfield CA1, thalamus, caudate nucleus, and the cerebellum. Lesions usually were less extensive in the I rather than NI members of a littermate pair, suggesting a possible protective effect of perinatal hypoxia against subsequent 3-NPA neurotoxicity.

Temporalspatial patterns of expression of metallothionein-I and -III and other stress related genes in rat brain after kainic acid-induced seizures

Kainic acid-induced seizures in the rat brain cause severe brain damage that is thought to result, in part, from oxidative stress. In this study, we examine the consequences of systemic administration of kainic acid on expression of several genes that encode proteins thought to play roles in protection from oxidative stress, including metallothionein-I, and -III. Kainic acid causes an increase in metallothionein-I and heme oxygenase-I mRNAs, as well as an increase in c-fos, heat shock protein-70, and interleukin-1 beta mRNAs. The induction of these mRNAs is seizure dependent, and is greater in brain areas with extensive damage (e.g. piriform cortex) than in areas with minimal damage (e.g. frontal cortex and cerebellum). In contrast, little or no change in mRNA for metallothionein-III, manganese superoxide dismutase, copper-zinc superoxide dismutase, glutathione-s-transferase ya subunit or glutathione peroxidase occur. The prolonged and robust concordant induction of the metallothionein-I and heme oxygenase-I genes may reflect the oxidative stress produced by kainic acid-induced seizures. In addition, the induction of interleukin-1 beta gene expression suggests an inflammatory response in brain regions damaged by kainic acid-induced seizures. Delineating the regulation of genes associated with oxidative and inflammatory responses can contribute to a fuller understanding of seizures and associated brain damage.